1. Field of Invention
The invention relates to a method and apparatus for determining the frequency of vibration and rate of energy loss for a stiff structure.
2. Discussion of Prior Art
Heretofore, considerable efforts have been devoted to developing nondestructive testing for evaluating the quality of stiff structures, such as lumber, products of paper and products of steel. The most widely used test is based on a flatwise bending test. Stiffness is measured by using a load-deflection relationship of a simple supported beam loaded at its mid-span. The modulus of elasticity of the structure or specimen is determined by measuring the bending deflection resulting from a known load.
A detailed theory of the mathematical principles of vibration testing are discussed in "A Vibrational Approach to . . . Nondestructive Testing of Structural Lumber", by Roy F. Pellerin, Forest Products Journal, Vol. XV3, March 1965, p. 93-101.
Additionally, U.S. Pat. No. 3,513,690 to Pellerin et al. discloses a method and apparatus for nondestructive testing of wood beams. The apparatus measures vibrations at the center of the span for a given wood specimen. Pellerin et al. utilizes a well known relationship between modulus of elasticity and the frequency of oscillation of a simply supported beam (Timoshenko, Young and Weaver, 1974). Drawbacks to this apparatus are that there is no adjustment or consideration for the difference between the span of the beam and the length of the beam. Also, the log decrement is determined using amplitude ratio of only two cycles and there are no computer or data logging parts. Additionally, Pellerin et al. does not measure the weight of the beam, only the vibration and vibration decay.
Recently, with the advancement of the personal computer, it has been possible to develop systems to perform fast fourier transform calculations to determine frequency of vibration. These FFT systems have greatly increased the capabilities of nondestructive testing of structural beams beyond the scope of the Pellerin et al. patent.
In early 1990, a nondestructive testing device for measuring the dynamic modulus of elasticity was developed. This data acquisition attachment, called DynaMOE.TM. requires a personal computer with an 80.times.87 math co-processor and an expansion slot. The device calculates the modulus of elasticity and a parameter relating to internal friction for softwood and hardwood timbers to determine strength properties. A weakness of this device is that it requires a computer with a math co-processor and an expansion slot. Additionally, the sampled data are restricted to 512 points and the user must trigger the data acquisition of the system.
The U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, published in August 1991, a paper titled, "Transverse Vibration Nondestructive Testing Using a Personal Computer", FPL-RP-502. The system utilized a lap-top personal computer to collect data from lumber specimens subjected to transverse vibrations. The data were used to compute dynamic modulus of elasticity using a flatwise bending test. Problems with this system relate to the analysis. The system is restricted to 512 Points, and the user must trigger the acquisition system. The system is only capable of displaying numerical results and specimens tested must have rectangular cross sections.
The nondestructive testing systems exemplified in the literature referred to hereinabove demonstrate many improvements made over the years in computerizing and improving the technology of transverse vibration testing systems. There still exists, however, in this industry, a need for a self-contained data acquisition which particularly improves the capabilities, efficiencies and economy of transverse vibration testing devices as well as simplifies installation and operation.